Reducing precision timing measurement uncertainty

ABSTRACT

Techniques for reducing precision timing message uncertainty are described herein. A method includes resetting an elastic buffer of a first device in response to a second device linked with the first device sending SKIP (SKP) ordered sets to the first device. The method also includes initiating a PTM handshake with the second device in response to resetting the elastic buffer. Additionally, the method includes sending PTM messages to the second device immediately after receiving the SKP ordered sets.

TECHNICAL FIELD

This disclosure relates generally to the Precision Timing Measurement mechanism. Specifically, this disclosure relates to Precision Timing Measurement uncertainty in connected devices.

BACKGROUND

One type of connection between devices is a peripheral component interface express (PCIe) connection. PCIe defines a Precision Timing Measurement (PTM) mechanism that allows two PCIe devices, a transmitter and receiver, to establish common timing through a sequence of repeated messages. These messages enable the devices to exchange timing or clock information. However, PCIe devices take the time measurement internally, after some amount of processing. Yet, the protocol defines time measurement as observed at the components' pins. Due to this difference in when the time is measured and when the time is considered to be measured, the accuracy of the PTM may be diminished by how much time passes between the time when packets pass over the pins, and when the packets are processed internally.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a fabric composed of point-to-point Links that interconnect a set of components;

FIG. 2 is a block diagram of a system for reducing precision timing message uncertainty;

FIG. 3 is a diagram of a clock signal for a local reference clock used in a system with spread spectrum clock frequency modulation;

FIG. 4 is a ladder diagram illustrating messages exchanged between linked components reducing precision timing message uncertainty; and

FIG. 5 is a method for reducing precision timing message uncertainty.

In some cases, the same numbers are used throughout the disclosure and the figures to reference like components and features. Numbers in the 100 series refer to features originally found in FIG. 1; numbers in the 200 series refer to features originally found in FIG. 2; and so on.

DESCRIPTION OF THE EMBODIMENTS

There is variation and uncertainty for applications that are trying to establish precise timing consistency between devices communicating across an interconnect fabric architecture. Accordingly, examples of the present techniques may reduce this uncertainty without changing the physical hardware implementations. Existing mechanisms for synchronizing the timing may be scheduled in a way that reduces the uncertainty in the transmit delay of PTM messages, when compared to standard implementations. This is also useful when links are operating in SRIS mode with small link widths in an interconnect fabric architecture.

One interconnect fabric architecture includes the Peripheral Component Interconnect Express (PCIe) architecture. One goal of PCIe is to enable components and devices from different vendors to inter-operate in an open architecture, spanning multiple market segments: Clients (Desktops and Mobile), Servers (Standard and Enterprise), and Embedded and Communication devices. PCIe is a high performance, general purpose I/O interconnect defined for a wide variety of future computing and communication platforms. Some PCI attributes, such as its usage model, load-store architecture, and software interfaces, have been maintained through its revisions, whereas previous parallel bus implementations have been replaced by a highly scalable, fully serial interface. The more recent versions of PCIe take advantage of advances in point-to-point interconnects, Switch-based technology, and packetized protocol to deliver new levels of performance and features. Power Management, Quality Of Service (QoS), Hot-Plug/Hot-Swap support, Data Integrity, and Error Handling are among some of the advanced features supported by PCIe.

In the following description, numerous specific details are set forth, such as examples of specific types of processors and system configurations, specific hardware structures, specific architectural and micro architectural details, specific register configurations, specific instruction types, specific system components, specific measurements/heights, specific processor pipeline stages and operation etc. in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details need not be employed to practice the present invention. In other instances, well known components or methods, such as specific and alternative processor architectures, specific logic circuits/code for described algorithms, specific firmware code, specific interconnect operation, specific logic configurations, specific manufacturing techniques and materials, specific compiler implementations, specific expression of algorithms in code, specific power down and gating techniques/logic and other specific operational details of computer system have not been described in detail in order to avoid unnecessarily obscuring the present invention.

Although the following embodiments may be described with reference to energy conservation and energy efficiency in specific integrated circuits, such as in computing platforms or microprocessors, other embodiments are applicable to other types of integrated circuits and logic devices. Similar techniques and teachings of embodiments described herein may be applied to other types of circuits or semiconductor devices that may also benefit from better energy efficiency and energy conservation. For example, the disclosed embodiments are not limited to desktop computer systems or Ultrabooks™. And may be also used in other devices, such as handheld devices, tablets, other thin notebooks, systems on a chip (SOC) devices, and embedded applications. Some examples of handheld devices include cellular phones, Internet protocol devices, digital cameras, personal digital assistants (PDAs), and handheld PCs. Embedded applications typically include a microcontroller, a digital signal processor (DSP), a system on a chip, network computers (NetPC), set-top boxes, network hubs, wide area network (WAN) switches, or any other system that can perform the functions and operations taught below. Moreover, the apparatus′, methods, and systems described herein are not limited to physical computing devices, but may also relate to software optimizations for energy conservation and efficiency. As will become readily apparent in the description below, the embodiments of methods, apparatus′, and systems described herein (whether in reference to hardware, firmware, software, or a combination thereof) are vital to a ‘green technology’ future balanced with performance considerations.

As computing systems advance, the components therein are becoming more complex. As a result, the interconnect architecture to couple and communicate between the components is also increasing in complexity to ensure bandwidth requirements are met for optimal component operation. Furthermore, different market segments demand different aspects of interconnect architectures to suit the market's needs. For example, servers require higher performance, while the mobile ecosystem is sometimes able to sacrifice overall performance for power savings. Yet, it is a singular purpose of most fabrics to provide the highest possible performance with maximum power saving. Below, PCIe is discussed, which potentially benefits from aspects of the present techniques described herein. Although the present techniques are not limited to PCIe, and other interconnect fabric architectures may also benefit from examples of the present techniques.

Referring to FIG. 1, an embodiment of a fabric composed of point-to-point Links that interconnect a set of components is illustrated. System 100 includes processor 105 and system memory 110 coupled to controller hub 115. Processor 105 includes any processing element, such as a microprocessor, a host processor, an embedded processor, a co-processor, or other processor. Processor 105 is coupled to controller hub 115 through a link 106, such as a front-side bus (FSB). In one embodiment, FSB is a serial point-to-point interconnect as described below. In another embodiment, link 106 includes a serial, differential interconnect architecture that is compliant with different interconnect standard than USB.

System memory 110 includes any memory device, such as random access memory (RAM), non-volatile (NV) memory, or other memory accessible by devices in system 100. System memory 110 is coupled to controller hub 115 through memory interface 116. Examples of a memory interface include a double-data rate (DDR) memory interface, a dual-channel DDR memory interface, and a dynamic RAM (DRAM) memory interface.

In one embodiment, controller hub 115 is a root hub, root complex, or root controller in a Peripheral Component Interconnect Express (PCIe or PCIE) interconnection hierarchy. Examples of controller hub 115 include a chipset, a peripheral controller hub (PCH), a northbridge, a southbridge, and a root controller/hub The memory controller is integrated with the processor 105. The controller 115 communicates with I/O devices, in a similar manner as described below. In some embodiments, peer-to-peer routing is optionally supported through root complex 115.

Here, controller hub 115 is coupled to switch/bridge 120 through serial link 119. Input/output modules 117 and 121, which may also be referred to as interfaces/ports 117 and 121, include/implement a layered protocol stack to provide communication between controller hub 115 and switch 120. In one embodiment, multiple devices are capable of being coupled to switch 120.

Switch/bridge 120 routes packets/messages from device 125 upstream, i.e. up a hierarchy towards a root complex, to controller hub 115 and downstream, i.e. down a hierarchy away from a root controller, from processor 105 or system memory 110 to device 125. Switch 120, in one embodiment, is referred to as a logical assembly of multiple virtual PCI-to-PCI bridge devices. Device 125 includes any internal or external device or component to be coupled to an electronic system, such as an I/O device, a Network Interface Controller (NIC), an add-in card, an audio processor, a network processor, a hard-drive, a storage device, a CD/DVD ROM, a monitor, a printer, a mouse, a keyboard, a router, a portable storage device, a Universal Serial Bus (USB) device, a scanner, and other input and output devices. Often in the PCIe vernacular, such a device, is referred to as an endpoint. Although not specifically shown, device 125 may include a PCIe to PCI or PCI-X bridge to support legacy or other version PCI devices. Endpoint devices in PCIe are often classified as legacy, PCIe, or root complex integrated endpoints. In one embodiment, the processor 105 is a transmitter (Tx), and the I/O device 125 is a receiver (Rx).

Graphics accelerator 130 is also coupled to controller hub 115 through serial link 132. I/O modules 131 and 118 also implement a layered protocol stack to communicate between graphics accelerator 130 and controller hub 115. Similar to the MCH discussion above, a graphics controller or the graphics accelerator 130 itself may be integrated in the processor 105.

In examples of the present techniques, the uncertainty in PTM message delay is reduced by scheduling PTM sequences immediately after sending SKIP (SKP) ordered sets. As part of the PCIe protocol, SKP ordered sets are periodically transmitted to compensate for differences in clock frequencies between the transmitter and receiver. Thus, the present techniques take advantage of the existing protocol, without taking up additional bandwidth.

FIG. 2 is a block diagram of a system 200 for reducing precision timing message uncertainty. The system 200 includes a device 202, with its own reference clock 204, and a device 206. It is understood that the PCIe protocol provides that each device 202, 206 acts as both a transmitter and receiver. As such, this simplified diagram is used to avoid confusion. However, in examples of the present techniques, each of the devices 202, 206 may represent either of a transmitter or receiver communicating with the other device over the PCIe connection.

The device 202 includes an elastic buffer 208, and the device 206 includes a PTM manager 210. The elastic buffer 208 is used in the receiver circuits of device 202 as part of the PCIe protocol to absorb differences between the rate of transmission of device 206 and the internal rate of reliever circuit processing in device 202. However, the accuracy of the timing of the PTM is impacted by variation in how long it takes for data packets to move through the elastic buffer 208. Because the amount of data in the elastic buffer 208 may vary over time, the amount of time it takes for packets to move through the elastic buffer 208 varies. This means that, even though the devices 202, 206 exchange clock information, the variation in the time it takes for this clock information to pass through the elastic buffer 208 may make the clock information less accurate. The PTM manager 210 coordinates the activities relating to the PTM protocol, including performing a PTM handshake between the devices 20, 206. By transmitting a series of SKP ordered sets before a PTM handshake, the PTM manager 210 places the elastic buffer 208 in a consistent state. Alternately, the PTM manager may be configured to schedule a PTM handshake to occur after a series of SKP ordered sets have been transmitted. In this way, the receipt of one or more SKP ordered sets resets the elastic buffer 208. The two arrows 212 below the elastic buffer 208 represent the idea that when a SKP ordered set is received, the elastic buffer 208 will go back to its neutral position 214. The neutral position 214 represents a consistent starting point that enables consistent timing for PTM messages. In the example elastic buffer 208, the neutral position 214 represents a halfway point, position 3 out of 5 total positions. In this example, the neutral position 214 is shown as being at the center of the elastic buffer 208, but in other implementations the neutral position is not required to be at the center of the elastic buffer. Because the elastic buffer 208 is in a consistent state at the neutral position 214 when the PTM messages are sent, the amount of time it takes to move through the elastic buffer 208 does not vary, in contrast to current systems.

Another issue can arise with PTM when the connection between the devices 202, 206, uses a clocking architecture with Independent SSC clocking, an example of which is the SRIS clocking architecture defined in PCIe. Spread-Spectrum Clocking (SSC) is an approach used to address potential electromagnetic interference from the reference clock 204. During a SSC modulation cycle, the energy of the clock signal is spread out over a number of frequencies. The modulated clock varies its frequency between two fixed frequency points over a set period of time. When the SRIS clocking architecture is used, the two devices 202, 206 use reference clocks that are allowed to be independent of each other. As such, the uncertainty due to the variation in the elastic buffer 208 is considerably increased, especially when narrow link widths are used in combination with large packet sizes, and can be up to several hundred nanoseconds.

FIG. 3 is a block diagram 300 of a clock signal 302 for a local reference clock used in a system with SSC frequency modulation. It is noted that the diagram 300 is a simplified and exaggerated example. As understood by one of ordinary skill in the art, the change in frequency is not as dramatic as shown. Further, the period of the modulation is much greater relative to the primary clock frequency than this example, which is provided for reasons of clarity.

As shown, the clock signal 302 varies in frequency over time 304. The arrows 306-1, 306-2, 306-3 represent the same phases in successive SSC modulation cycles. In an example of the present techniques, the PTM manager 210 schedules PTM message sequences to begin at the same phase in the SSC modulation cycle. In an example of the present techniques, clock circuits of the device 206 generate a pulse at the same phase in each clock modulation cycle, indicating to the PTM Manager 210 to initiate a PTM messaging, ensuring the elastic buffer 208 is in a consistent state when the PTM handshake is initiated. In this way, the latency effects of the elastic buffer 208 are relatively more consistent when the PTM messages are exchanged. This is because, in the the SRIS clocking architecture, the effects of SSC are the dominant factor affecting the latency through the elastic buffer 208. Thus, by causing the PTM exchanges to be initiated at the same phase in the SSC modulation cycle, the variability due to SSC is counteracted. It is noted however, the PTM message sequences may not be sent during every cycle. Rather, the PTM manager 210 may occasionally skip cycles based on the requirements of the specific PTM policy implemented. For example, PTM message sequences may be required only in every other cycle of the SSC modulation, in order to counteract a given amount of drift in a local time clock being synchronized.

In one example, these approaches can be applied unilaterally by the component initiating the PTM handshake, i.e., the transmitter or the receiver, based on its own reference clock. However, the approach can potentially work better in the case where it is known a-priori that the SSC period between the devices 202, 206 is matched, or otherwise correlated.

FIG. 4 is a ladder diagram illustrating messages exchanged between linked components reducing PTM uncertainty. As indicated at block 404, a transmitter 402, determines it is time to initiate a PTM handshake with a receiver 406. The transmitter 402 is a downstream port, and sends packets for one or more a SKP ordered sets, as indicated at 408. The SKP ordered sets are received at the receiver 406, which is an upstream port. The receiver 406 resets the elastic buffer 208, as indicated at 410. The transmitter 402 also sends PTM messages, as indicated at 412.

FIG. 5 is a process flow diagram of a method 500 for reducing precision timing messaging uncertainty. The method 500 is performed by the PTM manager 210 and the device 202, and begins at block 502, where the PTM manager 210 determines that it is time to initiate a PTM handshake between devices 202, 206. At block 504, the PTM manager 210 sends the SKP ordered sets to the device 202. At block 506, the device 202 resets the elastic buffer 208 in response to receiving the SKP ordered sets. At block 508, the PTM manager 410 initiates the PTM handshake between the devices 202, 206. At block 510, the PTM manager 210 sends PTM messages in response to the handshake from the device 202.

Both techniques reduce variation in the depth of the elastic buffer 208 of the device 202 when PTM messages are exchanged. Either method may be used depending on whether the SSC phase cycle information is available, and depending on whether the scheduling is flexible enough to be able to initiate PTM exchanges in close succession to the transmission of SKP ordered sets. It is also possible for additional improvement in the uncertainty of locally received PTM messages if the PTM manager 210 attempts to synchronize the transmission of SKP ordered sets to the expected time they are received from the link partner by observing the average interval of transmitted SKP ordered sets of the link partner.

EXAMPLES

An example includes a method for reducing precision timing measuring (PTM) uncertainty. The method includes resetting an elastic buffer of a first device in response to a second device linked with the first device sending SKIP (SKP) ordered sets to the first device. The method also includes initiating a PTM handshake with the second device in response to resetting the elastic buffer. Additionally, the method includes sending PTM messages to the second device immediately after receiving the SKP ordered sets. The method may include determining an average interval of the SKP ordered sets, and synchronizing transmission of the SKP ordered sets to an expected time based on the average interval. The method may include determining that it is time to initiate a PTM handshake, and sending the SKP ordered sets in response to the determination. The elastic buffer may be reset by setting the buffer at a neutral position. The neutral position may be a halfway point of the elastic buffer. The method may include generating a pulse from a reference clock indicating it is the time to initiate the PTM handshake. The method may include generating the pulse at a same phase of multiple cycles for a spread spectrum clock in an SRIS architecture.

An example includes an apparatus for reducing precision timing message uncertainty between linked devices. The apparatus includes a processor, an elastic buffer, and a memory comprising logic. According to the logic, the apparatus resets an elastic buffer of a first device in response to a second device linked with the first device sending SKIP (SKP) ordered sets to the first device. A PTM handshake is initiated with the second device in response to resetting the elastic buffer. PTM messages are sent to the second device immediately after receiving the SKP ordered sets. The apparatus may determine an average interval of the SKP ordered sets, and synchronize transmission of the SKP ordered sets to an expected time based on the average interval.

The apparatus of claim 8 determines that it is time to initiate a PTM handshake, sends the SKP ordered sets in response to the determination. The apparatus generates a pulse from a reference clock indicating it is the time to initiate the PTM handshake.

An example system for reducing precision timing message uncertainty, includes a transmitter device linked with a receiver device, the transmitter device comprising a transmitter reference clock, a transmitter elastic buffer; and

a transmitter memory, the transmitter memory comprising logic to reset the transmitter elastic buffer in response to receiving SKP) ordered sets from a linked device. A PTM handshake is initiated with the linked device in response to resetting the elastic buffer. PTM messages are sent to the linked device immediately after receiving the SKP ordered sets.

Not all components, features, structures, characteristics, etc. described and illustrated herein need be included in a particular embodiment or embodiments. If the specification states a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, for example, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element.

It is to be noted that, although some embodiments have been described in reference to particular implementations, other implementations are possible according to some embodiments. Additionally, the arrangement and/or order of circuit elements or other features illustrated in the drawings and/or described herein need not be arranged in the particular way illustrated and described. Many other arrangements are possible according to some embodiments.

In each system shown in a figure, the elements in some cases may each have a same reference number or a different reference number to suggest that the elements represented could be different and/or similar. However, an element may be flexible enough to have different implementations and work with some or all of the systems shown or described herein. The various elements shown in the figures may be the same or different. Which one is referred to as a first element and which is called a second element is arbitrary.

It is to be understood that specifics in the aforementioned examples may be used anywhere in one or more embodiments. For instance, all optional features of the computing device described above may also be implemented with respect to either of the methods or the computer-readable medium described herein. Furthermore, although flow diagrams and/or state diagrams may have been used herein to describe embodiments, the techniques are not limited to those diagrams or to corresponding descriptions herein. For example, flow need not move through each illustrated box or state or in exactly the same order as illustrated and described herein.

The present techniques are not restricted to the particular details listed herein. Indeed, those skilled in the art having the benefit of this disclosure will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present techniques. Accordingly, it is the following claims including any amendments thereto that define the scope of the present techniques. 

What is claimed is:
 1. A method for reducing precision timing measuring (PTM) uncertainty, the method comprising: resetting an elastic buffer of a first device in response to a second device linked with the first device sending SKIP (SKP) ordered sets to the first device; initiating a PTM handshake with the second device in response to resetting the elastic buffer; and sending PTM messages to the second device immediately after receiving the SKP ordered sets.
 2. The method of claim 1, comprising: determining an average interval of the SKP ordered sets; and synchronizing transmission of the SKP ordered sets to an expected time based on the average interval.
 3. The method of claim 1, comprising: determining that it is time to initiate a PTM handshake; and sending the SKP ordered sets in response to the determination.
 4. The method of claim 1, wherein resetting the elastic buffer comprises setting the buffer at a neutral position.
 5. The method of claim 4, the neutral position comprising a halfway point of the elastic buffer.
 6. The method of claim 1, comprising generating a pulse from a reference clock indicating it is the time to initiate the PTM handshake.
 7. The method of claim 6, comprising generating the pulse at a same phase of multiple cycles for a spread spectrum clock in an SSRI architecture.
 8. An apparatus for reducing precision timing message uncertainty between linked devices, the apparatus comprising: an elastic buffer; and a memory comprising logic to: reset an elastic buffer of a first device in response to a second device linked with the first device sending SKIP (SKP) ordered sets to the first device; initiate a precision timing measurement (PTM) handshake with the second device in response to resetting the elastic buffer; and send PTM messages to the second device immediately after receiving the SKP ordered sets.
 9. The apparatus of claim 8, the memory comprising logic to: determine an average interval of the SKP ordered sets; and synchronize transmission of the SKP ordered sets to an expected time based on the average interval.
 10. The apparatus of claim 8, the memory comprising logic to: determine that it is time to initiate a PTM handshake; and send the SKP ordered sets in response to the determination.
 11. The apparatus of claim 8, wherein the elastic buffer is reset by setting the buffer at a neutral position.
 12. The apparatus of claim 11, the neutral position comprising a halfway point of the elastic buffer.
 13. The apparatus of claim 8, the memory comprising logic to generate a pulse from a reference clock indicating it is the time to initiate the PTM handshake.
 14. The apparatus of claim 13, the memory comprising logic to generate the pulse at a same phase of multiple cycles for a spread spectrum clock in an SSRI architecture.
 15. The apparatus of claim 8, comprising a processor.
 16. A system for reducing precision timing message uncertainty, the system comprising: a transmitter device linked with a receiver device, the transmitter device comprising: a transmitter reference clock; a transmitter elastic buffer; and a transmitter memory, the transmitter memory comprising logic to: reset the transmitter elastic buffer in response to receiving SKP) ordered sets from a linked device; initiate a PTM handshake with the linked device in response to resetting the elastic buffer; and send PTM messages to the linked device immediately after receiving the SKP ordered sets.
 17. The system of claim 16, the memory comprising logic to: determine an average interval of the SKP ordered sets; and synchronize transmission of the SKP ordered sets to an expected time based on the average interval.
 18. The system of claim 16, the memory comprising logic to: determine that it is time to initiate a PTM handshake; and send the SKP ordered sets in response to the determination.
 19. The system of claim 16, wherein the elastic buffer is reset by setting the buffer at a neutral position.
 20. The system of claim 19, the neutral position comprising a halfway point of the elastic buffer.
 21. The system of claim 16, the memory comprising logic to generate a pulse from a reference clock indicating it is the time to initiate the PTM handshake.
 22. The system of claim 21, the memory comprising logic to generate the pulse at a same phase of multiple cycles for a spread spectrum clock in an SSRI architecture. 